In studies with macrophage cell lines we have selected and characterized mutants that do not show generalized phagocytosis. We have also studied phagocytosis of Salmonellae differing in pathogenicity for mice solely because of a difference in lipopolysaccharide (LPS) side chains, and shown that macrophage phagocytose them at a rate inversely proportional to pathogenicity by a complement-requiring mechanism. In studies with gram-negative outer membrane, we have shown that in several different gram-negative species and strains, LPS from a given strain contains from 0 to more than 40 difference side-chain lengths. In addition some Escherichia coli contain a long polysaccharide with the composition of LPS side chain but not attached to lipid A, loosely attached to the cell surface, that probably accounts for false capsule response. Many techniques that release LPS or "endotoxin" from the cell release copurifying proteins. We have now shown these to be outer membrane proteins and have quantitated several by antibody crossreactivity.